This invention relates to the handling of semiconductor wafers, and more particularly, to wafer handling systems and methods optimized for the holding and transfer of semiconductor wafers, such as thin semiconductor wafers.
The semiconductor industry requires automated wafer handling systems that can precisely handle semiconductor wafers or substrates, such as silicon wafers, of varying diameters, compositions and physical attributes and that can supply those wafers in a rapid, ordered succession to a wafer processing machine. Wafer handling generally entails removing each subject wafer from a wafer cassette, performing preprocessing operations, and individually loading the wafer into the wafer processing machine, followed by returning each processed wafer to a wafer cassette. In high-throughput wafer handling systems, wafers are manipulated or transferred among various locations within the wafer handling system by a robot having a transfer arm equipped with an end effector or a vacuum spatula that is adapted to manipulate and transfer. The wafer handling system typically includes a wafer aligning station for aligning and centering wafers and a load arm for inserting the centered and aligned wafers into a loadlock chamber of the wafer processing machine. The robot, transfer arm and end effector cooperate to place and remove wafers from the load arm, the wafer aligning station, and the wafer cassettes. To that end, the end effector, aligning station, and load arm have chucks, typically vacuum chucks, that securely hold the subject wafer in the vertical, horizontal, and inverted orientations required during the steps of the wafer-handling process.
A conventional chuck contacts an area on the backside or the device side of the wafer that suffices to provide a secure engagement. Certain conventional chucks restrict the contacted area to a narrow annular ring, termed the exclusion area, extending a predetermined distance radially inward from the peripheral rim of the wafer. The predetermined distance typically is less than or equal about 6 millimeters and is unpopulated by devices on the device side of the wafer. The utilization of the exclusion area permits the chucks of wafer handling systems to hold wafers by the device side without damaging devices that may be present during backside treatment and, during device side treatment, to hold wafers by the back side without disturbing any backside treatment. As a result, the exclusion area provides a 6-mm wide annular zone about the peripheral edge that chucks can safely utilize to reliably hold the wafer. One such wafer handling system is described in U.S. Pat. No. 5,820,329, hereby expressly incorporated by reference in its entirety herein.
Silicon wafers are typically manufactured according to standardized specifications which, among other dimensional tolerances, require the surface for receiving devices thereon to be substantially planar with a flatness of 1.5 microns or less. As-manufactured 150 mm silicon wafers, for example, have a standard diameter of 150xc2x10.2 mm and a standard thickness of 675xc2x125 microns. Silicon wafers are usually provided with a flat or a notch used for alignment and indicative of crystalline orientation. Although processed wafers vary in thickness depending upon the end product and the initial wafer diameter, a typical wafer thickness after processing ranges from about 500 microns to about 700 microns. The device fabrication process may introduce additional warpage or bowing that exceeds the flatness in an unprocessed state. For certain end applications, devices and integrated circuits are fabricated on thin silicon wafers having an average thickness of less than or equal to about 150 microns and exhibiting a warpage of as large as about 12.5 mm. Thin wafers are particularly useful in integrated circuit applications, for example, where the heat generated by devices during operation demands an enhanced thermal conduction through the thinned wafer to a heat sink, attached to the back side, in order to prevent overheating of the device side and loss or impairment of functionality.
The vacuum chucks of conventional wafer handling systems, under certain circumstances, are unable to engage the subject wafers in a fashion adequate for secure transfers. Conventional vacuum chucks cannot apply an adequate vacuum distribution over the exclusion area to be chucked. As a result, the wafer is not adequately secured for vertical and inverted orientations of the chuck and may even be dropped by the chuck during movement when the chuck is oriented horizontally. In particular, conventional wafer handling systems and vacuum chucks are unsuited for holding and transferring warped thin semiconductor wafers between locations within the system. In addition, the warpage of thin wafers hinders the accuracy of the alignment and centering operations performed by the aligning station. Therefore, conventional vacuum chucks employed in conventional wafer handling systems cannot apply an adequate gripping force such that thin wafers can be gripped or, if successfully gripped, manipulated about the wafer handling system without a significant risk of dropping the subject wafer.
Accordingly, there is a need for wafer handling systems and wafer handling techniques that can improve the handling of wafers and, in particular, thin wafers, by enhancing the ability of the vacuum chucks of the wafer handling system to secure and hold wafers during handling.
The present invention provides improved vacuum chucks and end effectors for a wafer handling device with an enhanced vacuum capability compared with conventional vacuum chucks and end effectors. The improved vacuum chucks and end effectors of the present invention are capable of handling common semiconductor wafers and specialized classes of semiconductor wafers, such as thin wafers. The improved vacuum chucks and end effectors have an increased conductance and segmented vacuum slots that assist in applying an attractive force sufficient to pull the exclusion are of the wafer to the contact the chuck or end effector and be secured thereto. As a result, wafer can be safely manipulated by the wafer handling system without a significant risk of either dropping or breaking the wafer.
According to the present invention, an end effector to hold and support a wafer comprises an effector body having a support surface for supporting the wafer thereupon and an internal vacuum plenum. The support surface comprises a plurality of vacuum ports arranged in a mutually spaced relationship. The internal vacuum plenum is connected for fluid communication with a vacuum source, which is operable for evacuating air from the internal vacuum plenum to thereby apply a subatmospheric pressure thereto. The vacuum plenum includes a plurality of flow diverters configured to divide the vacuum plenum into a plurality of vacuum distribution channels. At least one of the plurality of vacuum distribution channels is in fluid communication with at least one of the plurality of vacuum ports. The flow diverters are operable for redirecting the subatmospheric pressure to the vacuum distribution channels serving the vacuum ports unblocked by a portion of the wafer as each of said vacuum ports is occluded by a portion of the wafer.
According to the present invention, a vacuum chuck for holding and supporting a surface of a wafer comprises a chuck body with an outer periphery and an internal vacuum plenum of a predetermined height. A plurality of flow diverters are formed in the vacuum plenum. An annular rim is disposed about the outer periphery of the chuck body and defines a support surface for supporting a wafer thereon. Formed on the annular rim is a plurality of vacuum ports which are each in fluid communication with the vacuum plenum. A vacuum source is connected in fluid communication to the vacuum plenum and is operable for evacuating the vacuum plenum to a subatmospheric pressure. The flow diverters determine which of the plurality of vacuum ports, that are unobstructed, are to receive an increased subatmospheric pressure as the surface of the wafer partially engages the support surface and occludes certain of the plurality of vacuum ports.
According to the present invention, an apparatus for flattening a wafer comprises a source of a wafer-flattening gas, a wafer support, and a gas dispensing showerhead. The wafer support has a wafer supporting surface thereon adapted for contacting a first surface of the wafer. The showerhead comprising a tubular member connected for fluid communication with the source of wafer-flattening gas and a plurality of gas outlets positioned in a spaced relationship about the circumference of the tubular member, said tubular member suspended a predetermined distance above the surface of the wafer support, said plurality of gas outlets having a confronting relationship with the surface of the wafer support. The gas outlets are adapted to eject a flow of the wafer-flattening gas from the source against the first surface of a wafer supported on the wafer supporting surface for applying a force directed to press the second surface of the wafer against the wafer support surface.
Embodiments of the apparatus of the present invention may be provided in the form of a retrofit kit that includes an end effector and vacuum chuck of the present invention configured to replace the end effector and the vacuum chuck of the wafer aligning station of existing wafer handling systems and gas dispensing showerheads adapted to be installed adjacent to the vacuum chuck of the wafer aligning station and the vacuum chucks of the load arm.
According to the present invention, a method for securing a wafer against a support surface comprises positioning a first surface of a wafer proximate to the support surface and directing a flow of a wafer-flattening gas against a second opposite surface of the wafer for applying a force that presses at least a portion of the first surface of the wafer against the support surface.
According to the present invention, a method for securing a surface of a wafer to a support surface of a vacuum chuck comprises providing a plurality of vacuum ports in the support surface of the vacuum chuck which are in fluid communication with an internal vacuum plenum inside the vacuum chuck. The internal vacuum plenum is evacuated by a selectively-operable vacuum source. A vacuum pressure is applied to the vacuum plenum with the vacuum source and the surface of the wafer is positioned proximate to at least a majority of the plurality of vacuum ports. The surface of the wafer is allowed to occlude one of the vacuum ports and the vacuum pressure at the vacuum ports that remain unblocked is adjusted by diverting the vacuum pressure so that the unblocked vacuum ports receive a greater vacuum pressure and thereby apply a greater attractive force to the surface of the wafer.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.